This invention relates to butchering, particularly to an apparatus for scalding poultry.
After a chicken or other bird is slaughtered in a processing plant, it is carried by a leg shackle conveyor through a scalding bath, containing water at a temperature of about 128.degree. F. to 140.degree. F., to loosen the feathers prior to plucking. The immersion time is kept to the minimum necessary to loosen all the feathers, so as not to cook the outer layers of the bird, which can alter its color, among other effects. However, the feathers and anatomical irregularities both interfere with water flow around the bird and hence heat transfer, so that non-uniform scalding results. Two problem areas are around the tail, and the backs of the wings. To loosen the feathers sufficiently in the most difficult areas, it is necessary to keep the bird in the bath longer than is necessary for the better exposed areas.
When a bird is scalded, heat is transferred to the skin and feather follicles. Heat transfer rates are reduced by the feathers themselves, which are good insulators. Feathers track differently on different parts of the body. At some parts, for example the breast, feathers are easily penetrated by the scalding water, while at other locations, they are more difficult to penetrate. To get even scalding, it is necessary to enhance heat transfer at the difficult areas, e.g., the tail.
In an effort to improve heat transfer, and thus reduce scalding time, it is common to agitate the water around the bird, for example by injecting hot air into the scalding tank below the conveyor line. One example of this technique is described in U.S. Pat. No. 4,566,151.
In any scalding bath, a bird's buoyancy tends to bring it to the surface, so that part of the bird (for example, the problematic tail area) may be out of contact with the hot water. Air injection aggravates the buoyancy problem.
Whenever buoyancy causes incomplete scalding, rejection of the bird, or extensive hand picking, may follow. therefore, the buoyancy problem has been addressed previously, in a number of ways, including: (a) using a low-density scalding fluid like steam or hot air; (b) using water sprays instead of immersion--see, e.g., U.S. Pat. No. 2,412,338; (c) generating localized downward currents in a water bath sufficient to overcome buoyancy--U.S. Pat. No. 3,879,540; and (d) providing some sort of mechanical restrain above the bird. Mechanical devices previously used include rails at the surface of the bath, between which the feet of the bird pass. One such use is shown in U.S. Pat. No. 3,103,697. Techniques (c) and (d) are combined in U.S. Pat. No. 1,672,555.
A problem with physical restraints is that they generally constitute separate parts adding to the cost of the tank and requiring additional cleaning and service; they also obstruct service access to the inside of the tank. Furthermore, physical restraints above the tank provide an unnecessary opportunity for obstructing conveyors and other automated equipment. For these reasons, it would be preferable to have a scalding tank whose top was completely open, free of any obstruction.
Another problem, which arises in air-agitated scalding tanks, is that of flow interference. Each air injection head or diffuser forms a plume of bubbles which generate local water updrafts in the bath, above the diffusers, and local downdrafts between the diffusers. The plumes and drafts interfere with water flow along the length of the tank. To compensate for the increased flow resistance, a greater pressure gradient must be applied, resulting in large water level differences, from one end of the bath to another. These differences may be unacceptable, particularly if the bath is a long one.
We have found that, by raising the diffuser head substantially above the bottom of the bath, we can reduce plume-generated resistance to flow along the bottom of the tank.